Such fuel assemblies are used in PWRs, for instance. Some at least of the tubes form guides in which rods loaded with neutron absorbing material may be reciprocated. Such rods are connected in bundles to form control assemblies for fine adjustment of the reactivity of the reactor. In a particular embodiment, the grids and guide tubes are made of materials which are compatible and are welded together. In other embodiments, the grids are fixed to the guide tubes by deformation of sleeves threaded onto the guide and welded to the grid. In still another example (French Specification No. 2,088,009) the grids are axially slidable along the tubes for allowing axial expansion of the fuel elements during operation. Then the grids are provided with sleeves slidably received on the guide tubes. It has been suggested to include burnable or consumable poison in the fuel elements of a reactor (U.S. Pat. No. 3,625,821). A burnable poison is a material containing an element having a high neutron capture cross section which absorbs excess neutrons and does not give rise to strongly absorbent daughter products. Examples of such poisons are gadolinium, samarium, europium and, above all, boron (FR-A-No. 1,504,651).
Numerous solutions have already been proposed for incorporating the consumable poisons in a unit. One especially interesting embodiment (FR-A-No. 2 472 247) consists in placing in at least some of the guide tubes consumable poison rods comprising a casing in which annular pellets of poison (borated glass, for example) are immobilized between an end stopper and a deformation in the casing or in an internal tube. However, this arrangement has some disadvantages. Even after the poison is exhausted, the rods which contain it introduce a considerable mass of parasitic material which absorbs the neutrons and thus reduces the exploitation capacity of the fuel. The rod fills a guide tube and reduces the volume of the light water, thus also reducing the moderation, resulting in a disadvantageous use of the fuel when the reactor should be functioning with thermal neutrons. The poison rods and their associated structures occupy an important amount of storage space for long periods, since they cannot be subjected to re-processing.
It might be thought that the above disadvantages could be eliminated by coating the internal surface of some of the guide tubes with a substrate which holds a consumable poison. However, it should be borne in mind that each unit is generally used for a plurality of successive cycles and that, after the first cycle, the guide tubes may take rods appertaining to the control rods. The rubbing of the rods against the coating is then an important wear factor. Moreover, given that the guide tubes have a mechanical resistance function in the unit, it is not possible to carry out such treatment on the tubes without re-certifying them.